Phosphonomethylglycine Herbicides
Herbicide-tolerant plants may reduce the need for tillage to control weeds thereby effectively reducing soil erosion. One herbicide which is the subject of much investigation in this regard is N-phosphonomethylglycine, commonly referred to as glyphosate. Glyphosate inhibits the shikimic acid pathway which leads to the biosynthesis of aromatic compounds including amino acids, hormones and vitamins. Specifically, glyphosate curbs the conversion of phosphoenolpyruvic acid (PEP) and 3-phosphoshikimic acid to 5-enolpyravyl-3-phosphoshikimic acid by inhibiting the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (hereinafter referred to as EPSP synthase or EPSPS). For purposes of the present invention, the term “glyphosate” includes any herbicidally effective form of N-phosphonomethylglycine (including any salt thereof), other forms which result in the production of the glyphosate anion in plants and any other herbicides of the phosphonomethlyglycine family.
Tolerance of plants to glyphosate can be increased by introducing a mutant EPSPS gene having an alteration in the EPSPS amino acid coding sequence into the genome of the plant. Examples of some of the mutations in the EPSPS gene for inducing glyphosate tolerance are described in the following patents: U.S. Pat. No. 5,310,667; U.S. Pat. No. 5,866,775; U.S. Pat. No. 5,312,910; U.S. Pat. No. 5,145,783. These proposed mutations typically have a higher Kl for glyphosate than the wild-type EPSPS enzyme which confers the glyphosate-tolerant phenotype, but these variants are also characterized by a high Km for PEP which makes the enzyme kinetically less efficient (Kishore et al., 1998, Ann. Rev. Biochem. 57:627-663; Schulz et al., 1984, Arch. Microbiol. 137:121-123; Sost et al., 1984, FEBS Lett. 173:238-241; Kishore et al., 1986, Fed. Proc. 45: 1506; Sost and Amrhein, 1990, Arch. Biochem. Biophys. 282: 433-436). Many mutations of the EPSPS gene are chosen so as to produce an EPSPS enzyme that is resistant to herbicides, but unfortunately, the EPSPS enzyme produced by the mutated EPSPS gene has a significantly lower enzymatic activity than the wild-type EPSPS. For example, the apparent Km for PEP and the apparent Kl for glyphosate for the wild-type EPSPS from E. coli are 10 μM and 0.5 μM, respectively, while for a glyphosate-tolerant isolate having a single amino acid substitution of alanine for glycine at position 96, these values are 220 μM and 4.0 mM, respectively. A number of glyphosate-tolerant EPSPS genes have been constructed by mutagenesis. Again, the glyphosate-tolerant EPSPS had lower catalytic efficiency (Vmax/Km), as shown by an increase in the Km for PEP, and a slight reduction of the Vmax of the wild-type plant enzyme (Kishore et al., 1998, Ann. Rev. Biochem. 57:627-663).
Since the kinetic constants of the variant enzymes are impaired with respect to PEP, it has been proposed that high levels of overproduction of the variant enzyme, 40-80 fold, would be required to maintain normal catalytic activity in plants in the presence of glyphosate (Kishore et al., 1988, Ann. Rev. Biochem. 57:627-663). It has been shown that glyphosate-tolerant plants can be produced by inserting into the genome of the plant the capacity to produce a higher level of EPSP synthase in the chloroplast of the cell (Shah et al., 1986, Science 233, 478-481), which enzyme is preferably glyphosate-tolerant (Kishore et al., 1988, Ann. Rev. Biochem. 57:627-663).
The introduction of the exogenous mutant EPSPS genes into plant is well documented. For example, according to U.S. Pat. No. 4,545,060, to increase a plant's resistance to glyphosate, a gene coding for an EPSPS variant having at least one mutation that renders the enzyme more resistant to its competitive inhibitor, i.e., glyphosate, is introduced into the plant genome. However, many complications and problems are associated with these transgenic plants containing mutant EPSPS genes. Many such mutations result in low expression of the mutated EPSPS gene product or result in an EPSPS gene product with significantly lower enzymatic activity as compared to wild type. The low expression or low enzymatic activity of the mutated enzyme results in abnormally low levels of growth and development of the plant.
While such variants in the EPSP synthases have proved useful in obtaining transgenic plants tolerant to glyphosate, it would be increasingly beneficial to obtain a variant EPSPS gene product that is highly glyphosate-tolerant but still kinetically efficient, such that improved tolerance can be obtained with a wild-type expression level.
Recombinagenic Oligonucleobases
Recombinagenic oligonucleobases and their use to effect genetic changes in eukaryotic cells are described in U.S. Pat. No. 5,565,350 to Kmiec (Kmiec I). Kmiec I teaches a method for introducing specific genetic alterations into a target gene. Kmiec I discloses, inter alia, recombinagenic oligonucleobases having two strands, in which a first strand contains two segments of at least 8 RNA-like nucleotides that are separated by a third segment of from 4 to about 50 DNA-like nucleotides, termed an “interposed DNA segment.” The nucleotides of the first strand are base paired to DNA-like nucleotides of a second strand. The first and second strands are additionally linked by a segment of single stranded nucleotides so that the first and second strands are parts of a single oligonucleotide chain. Kmiec I further teaches a method for introducing specific genetic alterations into a target gene. According to Kmiec I, the sequences of the RNA segments are selected to be homologous, i.e., identical, to the sequence of a first and a second fragment of the target gene. The sequence of the interposed DNA segment is homologous with the sequence of the target gene between the first and second fragment except for a region of difference, termed the “heterologous region.” The heterologous region can effect an insertion or deletion, or can contain one or more bases that are mismatched with the sequence of target gene so as to effect a substitution. According to Kmiec I, the sequence of the target gene is altered as directed by the heterologous region, such that the target gene becomes homologous with the sequence of the recombinagenic oligonucleobase. Kmiec I specifically teaches that ribose and 2′-O-methylribose, i.e., 2′-methoxyribose, containing nucleotides can be used in recombinagenic oligonucleobases and that naturally-occurring deoxyribose-containing nucleotides can be used as DNA-like nucleotides.
U.S. Pat. No. 5,731,181 to Kimec (Kmiec II) specifically disclose the use of recombinagenic oligonucleobases to effect genetic changes in plant cells and discloses further examples of analogs and derivatives of RNA-like and DNA-like nucleotides that can be used to effect genetic changes in specific target genes. Other patents discussing the use of recombinagenic oligonucleobases include: U.S. Pat. Nos. 5,756,325; 5,871,984; 5,760,012; 5,888,983; 5,795,972; 5,780,296; 5,945,339; 6,004,804; and 6,010,907 and in International Patent No. PCT/US00/23457; and in International Patent Publication Nos. WO 98/49350; WO 99/07865; WO 99/58723; WO 99/58702; and WO 99/40789. Recombinagenic oligonucleobases include mixed duplex oligonucleotides, non-nucleotide containing molecules taught in Kmiec II and other molecules taught in the above-noted patents and patent publications.
U.S. Pat. No. 6,870,075 ('075 patent) discloses a method for producing a non-transgenic, herbicide resistant or tolerant plants employing recombinagenic oligonucleobases according to the methods disclosed in Kmiec I and Kmiec II. The EPSPS mutants disclosed in the '075 patent include changes made in the following amino acid positions of the EPSPS protein: Leu173, Gly177, Thr178, Ala179, Met180, Arg181, Pro182, Ser98, Ser255 and Leu198 in the Arabidopsis EPSPS protein or at an analogous amino acid residue in an EPSPS paralog.
Published US Patent Application 20030084473 also discloses the use of recombinagenic oligonucleobases to make non-transgenic herbicide resistant plants where the EPSPS protein has been changed in amino acid positions 126, 177, 207, 438, 479, 480 and/or 505 in the Arabidopsis EPSPS protein or at an analogous amino acid residue in an EPSPS paralog.
The present invention relates to additional amino acid mutations that can be made in any EPSPS gene from any species to produce a gene product that possesses resistance to glyphosate.